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Thread: Warm 147 amp and PT

  1. #1
    pp Pianissimo Drawbar Dave's Avatar
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    Warm 147 amp and PT

    On the 147 amp I *eventually* got finished, I have noticed that after an hour of use the amplifier gets very warm. The area of chassis above the relay, the side nearest the bass rotor and the output transformer are all warm. The power transformer is hotter than the rest. It is years since I owned a tube Leslie and can’t remember if this is normal heat transfer from the tube bases/cathode resistor.

    The amp works perfectly, sounds awesome and all the voltages are good.
    1937 Hammond BC/1970 Leslie 147
    1968 MK2 Farfisa Compact Duo/1964 Binson Echorec 2

    Restored and rehomed; RT3, C3, M102, L122, T202, 122, 147,760 x3

  2. #2
    pp Pianissimo Drawbar Dave's Avatar
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    After two hours the power transformer was as hot as a mug of coffee. I remember seeing waxy deposits underneath it when I was rebuilding the amp. The choke and output transformers were just as warm as the chassis from the tube heat, nowhere near as hot as the PT. I ordered the last 230 volt PT that Classic Hammond’s had in stock. At least I now know why the damn thing hadn’t been used for years!
    1937 Hammond BC/1970 Leslie 147
    1968 MK2 Farfisa Compact Duo/1964 Binson Echorec 2

    Restored and rehomed; RT3, C3, M102, L122, T202, 122, 147,760 x3

  3. #3
    ppp Pianississmo knife edge's Avatar
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    Hey Dave, are you sure there isn't something pulling too many Amps through the transformer? Maybe something shorted out?

    KE

  4. #4
    pp Pianissimo Drawbar Dave's Avatar
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    I got this amp as a supposed non runner that had not been used for a long time and no reason given. It also had no tubes. It has had a new set of tubes from my trusted supplier, full TWGH rebuild kit, a Trek relay and a replacement amphenol socket as the original was broken. There is nothing original other than the chassis and three transformers. As itís a 240 volt amp, the 117 volt motors are fed off the power transformer too. It still heats up with them unplugged too!

  5. #5
    ff Fortissimo muckelroy's Avatar
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    Most people on this forum probably won't have a convenient way of testing how hot their PTs get, because of the way the amp is installed in the Leslie, and that fact that the PT is in the very back of the amp, behind the already-hot 6550 tubes. While installed, you can't reach back there to feel it. I'm going to assume that you have the Leslie sitting out of the cab and/or on a work bench.

    I've done bench work on a large number of Leslie amps, and have never noticed the heat of the PT as being an issue. But then again, maybe it did get as hot as yours, I just didn't feel the need to touch it. When probing voltages, I have no need to touch the PT, though I have grasped the PT to move and re-position the chassis and have never been burned by it.

    You mentioned waxy deposits, and I assume that came from the old PT, and not the new PT. If instead you mean the new PT is producing waxy deposits, get a new PT please.

    Make sure the correct value fuse is installed in the Leslie. If nothing is smoking and the fuse is not blowing, carry on. When it fails, either your fuse will blow, or you just won't have power on one of the windings. If you are worried, I would source another PT, though I understand there are fewer 240V ones out there.

  6. #6
    pp Pianissimo Drawbar Dave's Avatar
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    The amp is actually in the Leslie! What alerted me to the possible issue was a hot smell. When I started work on this amp, the transformer lids were very rusty and flakey. I cleaned them off with my Dremel and painted them with Hammerite. When I finally got the BC and 147 joined up, I noticed after half an hour that there was a paint smell and investigated by unplugging the lower motor stack and very carefully sliding my hand along the side of the amp and feeling the temperatures. Much of the chassis gets warm from the tubes, as does the output transformer and choke, but the PT was much hotter after an hour, like a fresh cup of coffee, safe to hold but very hot.

    The fuse is a 1.5A Slo Blow and correct according to the manual for 117 and 240 volt versions.

    The amp functions as good as previous ones Iíve owned and all the voltages check out with the schematics.

    The dripping wax is on the original PT that is still in situ. The replacement one, a Hammond-Suzuki OEM one from Classic Hammonds is in the mail. Given the rarity of the 240 volt version I couldnít resist buying it, especially as it was the only one they had! If I donít need it yet Iíll sleep better knowing I have one in the workshop

    If anyone wants to check their own tube Leslie PT temperature and share it Iíd be grateful though as you say the fuse would catch any fault so maybe I should just leave it on for six hours and see what happens. Should they heat up to a certain degree? I might be worrying about nothing!

    Best
    Dave

  7. #7
    fff Fortississimo David Anderson's Avatar
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    Tube amp power transformers do get hot after being on for a while.

    The most definitive test is to check the current draw against known normal idle current draw for that particular amp. Of course, if the 230V version does also have the extra winding for the motors, that would increase the heating. You could unplug the motors and do a comparison. I don't have the figure memorized. The current figure for a 240V model should be one-half that of the 120V model: same power, double the voltage, halve the current. But that would exclude motor current.

    Some vintage HiFi amp power transformers get quite hot in extended operation, too hot to keep your hand on comfortably. It depends on where the manufacturer wanted to spend the money. The stock Dynaco ST-70 power transformer runs pretty hot, and as it heats up, you can watch the output voltage drop. You can buy an upgrade replacement from Triode Electronics that has a good bit larger lamination stack and runs much cooler -- and is stable in terms of output voltage.
    I'm David. 'Dave' is someone else's name.

  8. #8
    p Piano geoffbrown's Avatar
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    I assume that the breeze created by the bass rotor has a cooling affect on the amp,also looking at photo/s of the North American 122 and 147 leslie amps it appears that the PT has a higher lamination stack than the 230v models.

    As has been pointed out previously the 230v PT uses center tap on the primary winding to supply 115v to the motors,so I would expect the 230v PT to run hotter than the 115v version.
    My PR40 PT gets quite hot after running for a couple of hours.

    I will measure the temperature of my leslie amp PT and post

  9. #9
    pp Pianissimo Drawbar Dave's Avatar
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    Thanks for the reassurance guys. Iím probably worrying needlessly. When I was searching on this subject at the weekend I couldnít find anything other than Davidís post about an unfused amp that had been left on all night in a church and had burned up the PT. I donít own much tube gear any more so didnít have anything else to compare.

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